FLC OF A CANADAIR CL65 FAILED TO MAINTAIN AIRSPD GIVEN BY ARTCC CTLR DUE TO A MISUNDERSTANDING OVER THE 'IF ABLE' SPD.

Narrative

WHILE ON A VECTOR FOR SPACING; ZID ASSIGNED OUR FLT A SPD OF 250 KIAS FOR ADDITIONAL SPACING. ABOUT 5 MINS LATER; WE WERE GIVEN A DSCNT; CLRED TO THE BRICKYARD VOR (VHP); AND THEN TOLD 'IF ABLE DO 300 KIAS.' NEAR BRICKYARD VOR THE CTLR ASKED US OUR SPD; AT WHICH TIME WE SAID '250 KIAS.' THE CTLR THEN REPLIED; 'NO. YOU WERE ASSIGNED 300 KIAS.' WE THEN TOLD THE CTLR THAT OUR SPD WAS '300 KIAS IF ABLE.' THE CTLR; OBVIOUSLY CONFUSED AND FRUSTRATED; MADE A FEW NON PERTINENT REMARKS AND THEN ASSIGNED US A SPD OF 290 KIAS. WE BELIEVE THAT THIS EVENT STEMS FROM THE CTLR NOT REMEMBERING HIS INITIAL CLRNC FOR US. BY SAYING '300 KIAS IF ABLE;' THE CTLR IS LEAVING THE SPD TO THE FLC'S DISCRETION. ALSO CONTRIBUTING TO THE CTLR'S CONFUSION WAS THAT THERE WERE SEVERAL OTHER ACFT IN HIS SECTOR; ALL GOING TO THE SAME DEST; THAT WERE ALSO ON VECTORS AND SPD REDUCTIONS FOR SPACING. AT THE TIME; WE HAD ANOTHER PLT ON OUR JUMP SEAT THAT AGREED WITH OUR INTERP OF THE CLRNC. THE JUMP SEAT RIDER MADE A COMMENT ABOUT THE CTLR'S OBVIOUSLY OVERLOADED SIT WELL BEFORE THERE WAS ANY MISUNDERSTANDING OF AIRSPD ASSIGNMENTS.